Various methods and apparatus have been developed for fusing lead wires of coils to terminals, including methods and apparatus for fusing lead wires of coils of electric motor armatures to commutator bars. Fusing apparatus for connecting lead wires to commutator bars conventionally include a grounding electrode that engages the commutator bar at a point remote from the point at which the lead wire is to be secured to the commutator and a fusing electrode that engages the commutator at or quite close to the point of connection of the lead wire to the commutator bar.
When fusing commutators having lead-receiving hooks or tangs around which the lead wires are looped, the fusing electrode engages the radially outer portion of a tang and, during the fusing cycle when a high current is passed from the fusing electrode through the commutator bar to the grounding electrode, a generally radially-directed force is exerted on the tang by the fusing electrode, causing it to be bent into engagement with the bar substantially completely along the length of the tang, with the tang and the commutator bar in surrounding relation to a segment of the coil lead wire or lead wires looped around the tang. Based on empirical studies, it is known that the pressures applied to the tangs during the fusing cycle should be varied in a timed sequence throughout the duration of each fusing cycle. The optimum criteria for the degrees of pressures to be applied, for the sequences with which the pressures are changed, and for the times during which each pressure is applied will vary from one type of armature to another. Accordingly, it is common practice to undergo a series of trial sequences of force applied to the tangs of an armature of a particular construction until a sequence is determined that produces the fusion characteristics being sought for that particular armature. The fusing machine is then controlled in such a manner as to repeat the same sequence of pressure application for every armature of that particular construction.
Each fusing operation requires only a few line cycles to complete. Therefore, the mechanisms by which different pressure are applied to the tangs must have a good response time. ("Response time" as used herein refers to the time between which a signal is generated to initiate a change in pressure and the time the new pressure is applied.) Also, as a tang becomes heated during the fusing cycle, a stage is reached at which the tang acquires a plastic state and loses its resistance to the pressure applied by the fusing electrode. Optimally, the pressure applied to the tang by the fusing electrode will remain essentially constant. The characteristic of a fusing machine to operate to continue to exert a uniform pressure to a tang which has become heated to a plastic state is known as "force follow through." A machine having good force follow through will apply an essentially uniform preset or desired pressure to the tang even when the tang has been heated to become plastic.
To economically meet high speed production requirements, the fusing machine must be so designed that successive fusing operations are completed in rapid succession. The fusing machine must also be so designed that excessive pressures not be applied to tangs at any time. Of particular concern is that the fusing head, when it first engages a tang before the tang is heated by the fusing current, will do so with a force which is sufficiently low that the cold tang will not be bent. Otherwise, the tang may be bent in an uncontrolled manner which may not be repeated from one tang to the next, so that non-uniform and unreliably fused tangs and lead wires may result. For the same reason, when pressure is thereafter applied upon initiation of the fusing current, it is desirable that the pressure be evenly applied without an abrupt hammering of the tang, as would occur if the tang were struck or impacted against by a pressure applicator.
Prior fusing machines have used pneumatic or electromagnetic actuators or combinations thereof, either with or without associated spring devices, and electric motors, for applying forces to fusing heads to create the desired pressures between the tangs and the fusing heads. However, the known machines are relatively complex or expensive, or require exceedingly expensive controls, and many machines do not provide for adequate control of the forces exerted against the tangs. Many such machines do not exhibit good force follow through.